The overall objective of the proposed research is to delineate, and make sense of, the process of lexical development and its relations to communicative development, grammatical development, and both general and specific aspects of nonlinguistic cognitive development. The research will focus on the language and cognitive development of three groups: children with Williams syndrome, children with Down syndrome, and typically developing children. Out research during the current grant period indicates that the language skills of children with Williams syndrome are more advanced than their nonverbal cognitive skills; children with Down syndrome show the opposite pattern. Children who are typically developing tend to have equivalent levels of language and cognitive skills. Because of these differences in the general nature of the relations between language and nonlinguistic cognition, inclusion of all three groups in the same studies provides a unique opportunity to identify factors that are universal and necessary to lexical, communicative, or grammatical development versus those that are specific to particular rates or paths to language acquisitions. The proposed research consists of a 5-year longitudinal study beginning in late infancy and both longitudinal and cross-sectional studies involving preschool-and school-age children. Both observational and experimental methods will be used. There are four specific aims. The first is to delineate the nature of early lexical growth curves. The second is to further examine the role of lexical operating principles (heuristics) and foundational social cognitive abilities in the acquisition and extension of new words. The third is to begin to delineate the development of communicative abilities by toddlers and young children who have Williams syndrome. The fourth is to determine longitudinally the developmental trajectories of language and cognitive abilities for children with Williams syndrome. The research will have implications both for theoretical models of the relations between language and cognition and for the design of cognitive and language intervention strategies for children with developmental disabilities. [unreadable] [unreadable]